The present invention has the following objectives:
polycrystalline alkali-metal or alkaline-earth metal fluorides, produced in an original form, namely in the form of beads;
a process for the preparation (the conditioning) of said fluorides;
a process for the preparation of single crystals of the corresponding alkali-metal or alkaline-earth metal fluorides that uses polycrystalline fluorides in the aforementioned original form
providing an economic process for producing optical fluoride crystal raw material feedstock beads and manufacture of optical fluoride single crystals with optical fluoride crystal raw material feedstock beads.
The present invention was specifically developed for the preparation of fluoride single crystals, particularly optical fluoride single crystals and optical lithography element fluoride single crystal blanks and notably for single crystals of CaF2. Whereas its description more particularly references single crystals of CaF2, the invention is not limited to said crystals.
Single crystals of CaF2 are grown using the Stockbarger technique, which is familiar to those skilled in the art. They are the material of choice as optical element blanks for preparing UV (xcex less than 250 nm) optical lithography element lenses used in microlithography at 157 and 193 nm, as these lenses must have a below 200 nm high transmission, good laser resistance, low birefringence, and exhibit minimal fluorescence. These crystals must be prepared in the absence of water, air, and all sources of oxygen. This is well known to those skilled in the art. These single crystals are usually prepared in the presence of a fluoridating agent. PbF2 is the most commonly used fluorinating agent, as it is relatively easy to handle, is solid at room temperature, and has a high vapor pressure at the melting point of CaF2. Its reaction with CaO generates PbO, which is subsequently eliminated.
The process for growing large crystals of CaF2 (or any other alkali-metal or alkaline-earth metal fluoride) that are relatively free of defects typically lasts several weeks, particularly for high optical quality optical fluoride single crystals and below 200 nm optical lithography element fluoride single crystal blanks. The cost of the equipment as well as the staging of the crystal-growth process is significant and there is no guarantee of having a successful result at the end of the growth process. There has therefore long been a concerted effort to augment the yield of this crystal-growth process.
The problem of optimizing the mass of raw material feedstock introduced into the growth furnace with respect to the volume of said furnace has been faced. In practice, highpurity synthetic CaF2 is used as the raw material. This powder typically has an apparent density of only ca. 1.1 g/cm3, whereas the crystals produced have an apparent density close to the theoretical density, i.e. 3.18 g/cm3. Thus if synthetic powder is used directly as the raw material, ⅔ of the volume of the crystal-growth furnace is not used, or at least, not used efficiently. This same problem exists for all of the fluorides, as the crystals grown are denser than the raw material used.
An attempt has therefore been made to increase the density of said raw material before proceeding with the crystal-growth process. Increasing the density of the powder of a mineral salt typically entails its fusion and/or compression.
In the present case:
Compression is not a desirable solution: it requires special equipment, it is liable to introduce impurities, and the compressed masses produced can not be placed in intimate contact with a fluorinating agent before the start of and during the crystal-growth process. In any case, compression alone does not result in material having near theoretical density.
Nor is fusion a satisfactory technique: the products obtained from this process must be ground and grinding invariably affects the purity of the product.
Confronted with this technical problem, namely the optimization of the volume occupied by the raw material in the crystal-growth furnace and more generally, the optimization of the crystal growth process of alkali-metal and alkaline-earth metal fluorides, the inventors developed the original treatment process disclosed herein of said fluorides. Fluorides prepared by this method can be used for crystal growth in an optimized crystal-growth process.
The inventors propose an original procedure for increasing the density of the said fluorides. An advantageous variation of this original process assures both the densification and purification of these fluoride salts.
Said process of densification yields, starting from low-density and notably synthetic raw-material powder, beads with an apparent density that approaches the theoretical density of the corresponding fluoride. These beads have a small diameter. Therefore they do not need to be ground to be brought into efficient contact with a fluorinating agent when they are ultimately loaded into the crystal-growth furnace. Moreover, owing to their shape (quasi-perfect spheres), they can fill the crystal-growth crucible with minimal volume loss. The invention includes optical fluoride crystal raw material feedstock beads, the making of optical fluoride crystal feedstock beads and manufacture of optical fluoride single crystals and UV optical lithography element fluoride crystal blanks with optical fluoride crystal feedstock beads.
Those skilled in the art have certainly already understood the interest of the present invention. Its three aspectsxe2x80x94beads, their preparation, and their use in preparing single crystalsxe2x80x94are hereafter presented in detail.
As its first object, the present invention concerns a polycrystalline alkali-metal or alkaline-earth metal fluoride, prepared in the shape of beads. These beads have:
a diameter or equivalent diameter (their preparation procedure generates more or less perfect spheres) greater than or equal to 100 xcexcm, advantageously between 100 xcexcm and 2 cm; and
an apparent density greater than or equal to 60% of the theoretical density of the appropriate fluoride, advantageously at least 90% of the aforementioned theoretical density.
In the context of the invention, there is little interest in optical fluoride crystal feedstock beads of diameter (the word xe2x80x9cdiameterxe2x80x9d is used from now on to cover both the notion of diameter and equivalent diameter) less than 100 xcexcm or greater than 2 cm. More precisely:
beads with diameter less than 100 xcexcm are almost equivalent, in terms of density, to powder;
beads with diameter greater than 2 cm are not useful as such for the crystal-growth process. They must be ground to optimize packing and to increase their surface area to volume ratio (so they can be in intimate contact with the fluorinating agent). Such beads are equivalent to the xe2x80x9cpiecesxe2x80x9d obtained from the compression and/or fusion of a powder.
The beads of this invention have an apparent density greater than their corresponding powders. Their apparent density tends toward the theoretical density of the constituent material. The beads of this invention have an apparent density at least equal to 60%, advantageously at least equal to 90%, of the theoretical density of the fluoride in question. Therefore, beads of CaF2 produced according to the present invention have a density greater than or equal to 1.9 g/cm3, advantageously greater than or equal to 3 g/cm3.
The term xe2x80x9capparent densityxe2x80x9d is familiar to those skilled in the art. The density of a material is constant under given conditions of temperature and pressure. The term xe2x80x9capparent densityxe2x80x9d is referred to for a solid material dispersed in the form of particles in such a way that its xe2x80x9cdensityxe2x80x9d is sensitive to the size and surface state of said particles. In the present context, the apparent density of powders or beads can be defined as the mass of said powders or beads (g) that can be placed into a given volume (cm3), at ambient temperature and without application of pressure.
The optical crystal feedstock densified beads of the invention constitute an excellent solution to the technical problem set forth above: namely the optimization of the mass of raw material introduced in the crystal-growth furnace with respect to the volume of said furnace. In a given volume a greater quantity of raw material pre-conditioned in the form of dense beads can be introduced than of powder of the same raw material. Beads of several different diameters can be produced and by tailoring this distribution of diameters the mass of raw material introduced in a given volume can be further increased (with respect to the mass of raw material in the form of beads all having the same diameter).
In any case, the beads of the invention are more easily manipulated than a powder.
The beads densified by an advantageous variant of the invention have an oxygen content less than 200 ppm, advantageously less than 50 ppm. To obtain such beads, a preparation procedure (the densification procedure with a preferred innovation described in detail below) is implemented under optimum conditions for protection from oxygen and moisture. Concomitant densification and purification of the desired alkali-metal or alkaline-earth metal fluoride is therefore achieved.
The beads of the invention, which advantageously have a low oxygen content, more particularly consist of beads of lithium fluoride (LiF), magnesium fluoride (MgF2), calcium fluoride (CaF2) barium fluoride (BaF2), and especially beads of calcium fluoride (CaF2).
A new densified, and advantageously purified, form of alkali-metal or alkaline-earth metal fluorides is thus proposed according to the present invention, that can be directly used as optical crystal feedstocks to prepare single crystals of said fluorides. This new densified feedstock form can be obtained by the process described hereafter, which constitutes the second object of the present invention. This process characteristically comprises:
the preparation of a molten mass of an alkali-metal or alkaline-earth metal fluoride;
allowing said molten mass to flow through at least one orifice opening into a space; a significant difference being maintained between the temperature of said molten mass and that of said space into which the molten mass is introduced such that at the outlet of said orifice(s) the molten mass is divided into droplets;
the progressive solidification of said droplets as they fall along a vertical axis;
recovering said totally solidified droplets in the form of beads; each of these steps being carried out in the absence of humidity and oxygen (in order to obtain relatively pure beads).
This process can be summarized as the generation of droplets from a molten mass followed by the solidification of these droplets as they pass through a steep thermal gradient. This process is not per se original. It is original by way of its field of application and the resultant mode of operation.
The molten mass is generally obtained from a powder, notably a high-purity synthetic powder. The molten mass can also be prepared from granules.
The diameter of the orifice(s) is obviously dictated by the desired bead diameter. An orifice diameter approximately 75% that of the desired bead diameter can be recommended. In any case the most influential parameters for bead diameter are:
the diameter of said orifice(s),
the temperature and the pressure applied to the molten mass.
The diameter of the orifice or orifices has been mentioned. This use of the singular article is not limiting. It is entirely conceivable, even judicious, that the orifices not all have the same diameter. In this way, beads of different diameters can be prepared during the pouring of one molten mass in a single process.
Beads, with different diameters xcfx861 . . . xcfx86n, nxe2x89xa72, can be prepared in a single run (using a single crucible with orifices of corresponding diameters d1, . . . dn), or in successive runs (using n crucibles each having orifices of a single given diameter d1, . . . dn). Such beads can advantageously be mixed and used to load a crystal-growth furnace.
With respect to the generation of droplets and their solidification, one skilled in the art is apt to optimize the thermal gradient and the height of the solidification zone. The molten mass must obviously be brought to a temperature at which it is homogeneous (for example, a temperature greater than 1380xc2x0 C. for CaF2). The thermal gradient must also be sufficient to obtain the desired effect (for CaF2, a gradient of at least 200xc2x0 C. between the temperature of the molten mass and the level where the beads are recovered).
The process of the invention, as described above, is advantageously carried out at reduced pressure. Under such conditions the (virtual) absence of all contaminants (H2O, O2, . . . ) can be ensured and non-reacted species are removed, as well as the by-products of the reaction with a fluorinating agent, when such an agent intervenes.
The intervening atmosphere, advantageously at reduced pressure therefore, can be an inert atmosphere. An argon atmosphere is a possible choice. It is advantageous that the atmosphere be fluorinating. In generating beads in such a fluorinating atmosphere, their constituent material is purified.
There are several ways in which a fluorinating atmosphere can be generated in the vessel in which the beads are produced: at least one fluorinating agent intervenes in the molten mass and/or in the space through which the droplets fall (i.e. the solidification zone). Said fluorinating agent can be introduced in the solid state, advantageously mixed with the raw material, in the molten mass. The fluorinating agent can also be introduced as a gas, advantageously in the solidification zone. It is in no way excluded to introduce the gaseous fluorinating agent above the molten mass, the solid agent in the solidification zone, and the gaseous and/or solid agent(s) in the receptacle containing the molten mass and in the solidification zone. Said agent, in the solid form, is advantageously selected from PbF2, NH4F, NH4F.HF and polytetrafluoroethylene (Teflon(copyright)). Advantageously, PbF2 is used. HF, F2, and NF3 are the preferred gaseous fluorinating agents.
Another advantageous characteristic of the process of the invention is insisted upon when carried out as described above, namely with the introduction of a fluorinating agent. As the droplets formed have a small volume, and as they solidify progressively, the said fluorinating agent (PbF2, for example) has enough time to react and the reaction products (PbO in this case) have ample time to escape from said droplets. The purification is thus performed under optimum conditions.
According to a preferred variant, the bead-preparation process is carried out under conditions such that the pressure in the vessel containing the molten mass is greater than that in the solidification zone. This pressure can be realized using an inert or a fluorinating gas. The pressure serves to accelerate the rate at which the molten material drips through the orifice or through each orifice. It also influences the diameter of the droplets.
The process of this invention is obviously carried out in an adequate vessel, constructed of material(s) capable of withstanding fluoride attack. The vessel comprises:
a crucible capable of being heated adequately and that can be used to contain the molten mass; the bottom of this crucible has at least one orifice;
a chamber into which said orifice(s) feed(s); means of heating and/or cooling said chamber being associated to said chamber for generating the desired thermal gradient along its axis.
In general the beads are collected in a receptacle placed at the bottom of the chamber.
Said crucible and said chamber can be arranged in several ways. In one variant, the crucible is contained in a compartment above the chamber. In another variant, the crucible is located in the upper part of the chamber. The latter variant is preferred.
To ensure the control of the atmosphere in the crucible, the process is obviously carried out in an enclosed system.
One skilled in the art is able to design an apparatus capable of generating beads according to the process described above. Such an apparatus is described later on in this text with reference to the attached FIGURE.
To retain their purity, the beads should be stored in the absence of air and moisture.
It has been seen that the feedstock beads constitute the raw material of choice for carrying out the optical crystal-growth process. As a third object, the present invention thus relates to such a process; that is a process of preparing an alkali-metal or alkaline-earth metal fluoride single crystal optical blank by the crystallization of the corresponding polycrystalline fluoride into an optical element blank. According to the invention the polycrystalline fluoride is pre-conditioned in the form of feedstock beads such as those described above.
All of the inherent advantages of these beads have been insisted upon.
It has also been indicated that the feedstock beads constituting the charge of an optical crystal-growth furnace are not required to be of the same diameter. With beads of the same diameter, significant optimization of the ratio
R=mass of raw material loaded into the furance/furnace volume,
is already achieved.
With beads of different diameter, this ratio can be further increased; smaller beads can fill the voids between the larger beads.
Thus, an advantageous variant of the present invention is to grow crystals using beads of different diameters (at least two different diameters).
As the crystal-growth furnace is rarely completely free from traces of oxygen and/or sources of oxygen (such as H2O), it is strongly recommended to carry out the optical crystal growth in the presence of at least one fluorinating agent, advantageously in the presence of PbF2.
Therefore, an effective quantity of PbF2 can be directly mixed with the beads of this invention, the resultant mixture being a convenient charge for the crystal-growth crucibles. It is re-emphasised here that there is no need to grind said beads.
The crystal-growth process as described above is advantageously performed for the preparation of UV optical lithography single crystals of CaF2 from polycrystalline CaF2 feedstock beads.